Writing a Simple Operating System — from Scratch

CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL

MODE) 21

some_function:

pusha ; Push all register values to the stack

mov bx, 10

add bx, 20

mov ah, 0x0e ; int =10/ah=0x0e -> BIOS tele -type output

int 0x10 ; print the character in al

popa ; Restore original register values

ret

3.4.7 Include Files

After slaving away even on the seemingly simplest of assembly routines, you will likely
want to reuse your code in multiple programs. nasm allows you to include external files
literally as follows:

%include "my_print_function.asm" ; this will simply get replaced by

; the contents of the file

mov al, ’H’ ; Store ’H’ in al so our function will print it.

call my_print_function

3.4.8 Putting it all Together

We now have enough knowledge about the CPU and assembly to write a more sophisti-
cated “Hello, World” boot sector program.

Question 4

Put together all of the ideas in this section to make a self-contained function for printing
null-terminated strings, that can be used as follows:

;

; A boot sector that prints a string using our function.

;

[org 0x7c00] ; Tell the assembler where this code will be loaded

mov bx, HELLO_MSG ; Use BX as a parameter to our function , so

call print_string ; we can specify the address of a string.

mov bx, GOODBYE_MSG

call print_string

jmp $ ; Hang

%include "print_string.asm"

; Data

HELLO_MSG:

db ’Hello , World!’, 0 ; <-- The zero on the end tells our routine